I had to troubleshoot an Apache installation recently. Apache 2.0 was running on several Linux boxes behind a load balancer. If you ran top on each box, the CPU was mostly idle, there was plenty of memory available, and yet Apache seemed sluggish. Here are a couple of things I did to speed things up.
1. Disable RedirectMatch directives temporarily
All the Apache servers had directives such as:
RedirectMatch /abc/xyz/data http://admin.mysite.com/abc/xyz/data
This was done so administrators who visited a special URL would be redirected to a special-purpose admin server. Since the servers were pretty much serving static pages, and they were under considerable load due to a special event, I disabled the RedirectMatch directives temporarily, for the duration of the event. Result? Apache was a lot faster.
2. Increase MaxClients and ServerLimit
This is a well-known Apache performance optimization tip. Its effect is to increase the number of httpd processes available to service the HTTP requests.
However, when I tried to increase MaxClients over 256 in the prefork.c directives and I restarted Apache, I got a message such as:
WARNING: MaxClients of 1000 exceeds ServerLimit value of 256 servers, lowering MaxClients to 256. To increase, please see the ServerLimit directive.
There is no ServerLimit entry by default in httpd.conf, so I proceeded to add one just below the MaxClients entry. I restarted httpd, and I still got the message above. The 2 entries I had in httpd.conf in the IfModule prefork.c section were:
At this point I resorted to all kinds of Google searches in order to find out how to get past this issue, only to notice after a few minutes that the number of httpd processes HAD been increased to well over the default of 256!
UPDATE 03/06/09: It turns out that the new MaxClient and ServerLimit values take effect only if you stop httpd then start it back again. Just doing a restart doesn't do the trick...
So, lesson learned? Always double-check your work and, most importantly, know when to ignore warnings :-)
Now I have a procedure for tuning the number of httpd processes on a given box:
1. Start small, with the default MaxClients (150).
2. If Apache seems sluggish, start increasing both MaxClients and ServerLimit; restart httpd every time you do this.
3. Monitor the number of httpd processes; you can use something like:
ps -def | grep httpd | grep -v grep | wc -l
If the number of httpd processes becomes equal to the MaxClients limit you specified in httpd.conf, check your CPU and memory (via top or vmstat). If the system is not yet overloaded, go to step 2. If the system is overloaded, it's time to put another server in the server farm behind the load balancer.
That's it for now. There are many other Apache performance tuning tips that you can read from the official Apache documentation here.